1. Field of the Invention
This invention relates generally to wireless telephones. More particularly, it relates to a linearization technique allowing use of a non-linear power amplifier in a time division multiple access (TDMA) wireless telephone.
2. Background of Related Art
Mobile wireless telephones have become commonplace in today""s society. Wireless telephones enable a user to participate in a telephone call virtually wherever they go.
Initially, wireless telephones communicated with a base station using analog transmission techniques. However, more recently digital techniques such as time division multiple access (TDMA) have been implemented in wireless telephones. TDMA generally refers in the wireless telephone world as a technology for delivering digital wireless service using time division multiplexing (TDM). TDMA works by dividing a radio frequency into time slots, and then allocating time slots to mobile telephones as the needs arise. In this way, a single frequency can support multiple, simultaneous data channels.
FIG. 1 is a block diagram of the relevant portion of a conventional TDMA wireless telephone.
In particular, in FIG. 1, a modulated signal is input to a variable gain amplifier 502, and then to a power amplifier 504 for transmission. A portion of the signal output from the power amplifier 504 is detected by an envelope detection circuit 506, and the detected signal is output to a correction circuit 508. The correction circuit 508 corrects the input/output (I/O) characteristics of the variable gain amplifier 502 based upon characteristics in the non-linear area of a detecting diodes to characteristics approximate to linear characteristics by the use of characteristics reversed to the characteristics of the non-linear area of the power amplifier 504.
Generally, the power amplifier 504 provides the energy necessary to transmit a wireless signal to a receiving base station some distance away. In a wireless device, the power amplifier 504 is a major contributor (and usually the most significant contributor) to the drain of the battery of the device. Increased efficiency in the power amplifier 504 leads directly to increased talk time, to increased standby time, and/or to reduced weight (i.e., smaller battery) and/or to decreased size of the wireless device.
While advantages can be gained by reduced linearity of the power amplifier 504, there are limitations. For instance, current industry standards specify the maximum distortion level of the transmitted output signal. Conventionally, a linear power amplifier is used to insure that the output signal is clear and undistorted. The higher the level of the power amplifier""s linearity, the lower the distortion level of the transmitted signal. However, linearity and efficiency are inversely related to one another. Thus, a high efficiency power amplifier usually is a non-linear power amplifier.
Clearly, it is beneficial to reconcile the balanced relationship between the requirements for linearity and efficiency.
One technique for improving the linearity of a power amplifier without sacrificing efficiency is by introducing linearization circuits to a non-linear amplifier. The assumption behind this idea is that linearizators consume much less DC power than linear power amplifiers. One conventional implementation of linearization circuits is the use of a xe2x80x9cfeed forwardxe2x80x9d linearization technique. A feed forward linearization technique is shown in, e.g., Japanese Patent Application number 01713661 to OKI Electric Ind. Co. Ltd, published Jan. 21, 1997.
Unfortunately, conventional linearization methods require additional circuitry, which increases the cost and reliability of a wireless transmitter. Moreover, such conventional linearization techniques have typically not been comprehensively tested for applications to Class C amplifiers.
Thus, there is a need for a simple and inexpensive technique and apparatus for improving the efficiency of a wireless device transmitter, particularly in a TDMA transmitter, without sacrificing linearity of the power amplifier in doing so.
In accordance with the principles of the present invention, a transmitter of a wireless device comprises a power amplifier, and a root raised cosine filter. A first set of parameters for the root raised cosine filter adapt the root raised cosine filter to provide a first level of linearization to a modulated signal amplified by the power amplifier. A second set of parameters for the root raised cosine filter adapt the root raised cosine filter to provide a second level of linearization to the modulated signal amplified by the power amplifier.
A method of improving efficiency of a power amplifier in a transmitter in accordance with another aspect of the present invention comprises adaptively configuring a first set of parameters in a root raised cosine filter when a transmit power of the power amplifier is below a predetermined level, and adaptively configuring a second set of parameters in the root raised cosine filter when a transmit power of the power amplifier is above the predetermined level.